The present invention relates to rocket motors and in particular to coatings for improving the mechanical strength of the propellants used in the motors.
Although the present invention can be used beneficially with a number of propellant compositions, it has been found to be of particular value when used with the so-called XLDB and CMDB propellants defined in the foregoing Abstract. Although XLDB propellant is used extensively and used to advantage in many rocket motors, such as the Trident C-4, its composition nevertheless is known to involve a trade off in that its increased energy content is obtained by some sacrifice in its mechanical strength. Increasing its strength while preserving its energy level is regarded as a matter of significant advantage.
An XLDB propellant formulation includes finely-divided solid oxidizers, such as cyclotetramethylene tetranitramine (HMX), ammonium perchlorate (AP) and metal fuels bound together by a binder system consisting primarily of a polymeric system and a plasticizer. The polymeric system contains nitrocellulose (NC) as an energetic component, a liquid polyglycol adipate (PGA) as a non-energetic component and hexamethylene diisocyanate (HDI) as a curing agent. The plasticizer usually is nitroglycerin (NG) or other nitrate esters. The binder also contains small amounts of stabilizers such as nitrodiphenylamine and N-methyl-p-nitroaniline, or other nitroanilines and very small amounts of a cure catalyst such as triphenyl bismuth or organic tin salts.
The principle advantage of these propellants when compared with other composite propellants is its increase in specific impulse due to the increased energy provided by the nitrate ester plasticizer. However, in compositions containing large amounts of NG plastizer, propellant strengths, such as tensile and tear strengths and modulus, tend to be relatively low. The reason is that these strengths are decreased because nitrate ester or NG is added at the expense of the polymeric binder. A trade off therefore involves specific impulse and propellant strength.
An object of the present invention is, therefore, to improve the propellant strength without sacrificing the energy level provided by the propellant formulation.
More specifically, another object which will become more apparent in the ensuing description, is to provide a particular coating for the propellant which improves its tear strength without sacrificing its energy content or significantly affecting the ignition capability of the propellant.
A further object is to improve the tear strength by providing a coating for selected areas of the port surfaces of an XLDB propellant.